This is a mated pair of neon gobies, Gobiosoma oceanops from Florida.
Please Note: Any new fish purchased from us or from a local pet store should to be quarantined. All fish from anywhere in the wild can be possible carriers of bacteria and protozoa that can lead to an infection in your system, so we always recommend that you use some sort of quarantine system prior to adding them to your system. If you have a fish only system and can medicate the whole system, you may not need a separate quarantine tank. If you have a reef system that cannot be medicated, a good ultraviolet sterilization system should prevent any kind of disease outbreak. We medicate our system for bacterial infections and protozoans, but because we don't always hold our fish for long periods of time, there is no way to be sure all the protozoan cysts have been killed. A little bit of prevention will save you lots of trouble down the line.